Primitivity, Uniform Minimality, and State Complexity of Boolean Operations
A minimal deterministic finite automaton (DFA) is uniformly minimal if it always remains minimal when the final state set is replaced by a non-empty proper subset of the state set. We prove that a permutation DFA is uniformly minimal if and only if its transition monoid is a primitive group. We use this to study Boolean operations on group languages, which are recognized by direct products of permutation DFAs. A direct product cannot be uniformly minimal, except in the trivial case where one of the DFAs in the product is a one-state DFA. However, non-trivial direct products can satisfy a weaker condition we call uniform Boolean minimality, where only final state sets used to recognize Boolean operations are considered. We give sufficient conditions for a direct product of two DFAs to be uniformly Boolean minimal, which in turn gives sufficient conditions for pairs of group languages to have maximal state complexity under all binary Boolean operations (“maximal Boolean complexity”). In the case of permutation DFAs with one final state, we give necessary and sufficient conditions for pairs of group languages to have maximal Boolean complexity. Our results demonstrate a connection between primitive groups and automata with strong minimality properties.
KeywordsState complexity Finite automaton Primitive group Uniformly minimal Boolean operation Transition monoid
I thank Jason Bell, Janusz Brzozowski and an anonymous referee for careful proofreading and helpful comments. The computer algebra system GAP  was invaluable for this research; I cannot overstate its importance in obtaining these results. In particular, I thank the authors of the Automata GAP package  and all contributors to GAP’s library of primitive groups.
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